A transmission system refers in the present context to a modulation scheme at the transmitter, the spectral and temporal characteristics of the light, the physical path of transmission including the fibre, optical amplifiers, filters isolators and any other components, and the receiver. Although the components can be individually characterised, their response as a system is not a trivial function of the individual characteristics. The over-riding reason for this is the interplay between non-linearity and dispersion effects which become important in long-haul transmission systems. For long distance repeaterless systems, the main constraint on transmission distance is the loss in the system due to the constant attenuation of the fibre, and dispersion due to the chirping of the transmitter. These systems can often be considered as being linear in their response, and the transmission over the optical fibre can be treated as simply adding a certain amount of dispersion, and attenuating by a determined amount.
Semiconductor laser diodes are almost universally used as the transmitter in current and proposed fibre optic communications systems. Most systems employ some form of digital communication whereby the signal which is being transmitted is represented as a sequence of 1's and 0's, independent of nature of the information being transported. The way in which these 1's and 0's are represented in a transmission system depends upon the type of modulation employed.
The most common type of modulation is known as Amplitude Modulation, where the intensity of the light indicates the state of a particular bit of information, high intensity for 1's and low intensity for zeros. This is usually achieved by turning the laster on and off by modulating the current to the laser.
Two formats are used commonly, Return to Zero (RZ), where the laser is turned off after each bit of information, usually for the same length of time that the bit was on, and Non Return to Zero (NRZ), where the intensity is constant for the whole period allocated to the bit and will only change when there is a change from one bit to the next. This is shown in FIG. 2.
One result of amplitude modulation is that the wavelength (and hence frequency) of the light is chirped, i.e. the wavelength of the laser changes during the transmission of the bit because the laser transmission wavelength is a function of the current applied. This effect is detrimental to long distance transmission because optical fibre is dispersive, i.e. different wavelengths travel at slightly different speeds down the fibre. As a result, the light from one bit arrives at the receiver at different times, and so is distorted and can interfere with the other bits. This effect limits the distance that data can travel down an optical fibre, and becomes increasingly important at high bit rates.
An alternative to modulating the current to the laser diode is to apply a constant current to the laser diode, and to externally modulate the light from the laser using a device such as a lithium Niobate crystal modulator. This effectively reduces the chirp, but is difficult to realise because of the high drive voltages required. Additionally, Brillouin scattering can be a problem over long distances because of the large wavelength component at a single frequency.
Several other schemes have been proposed which are advantageous to long distance transmission, among which are coherent techniques, frequency shift keying (FSK) and soliton transmission. Frequency Shift Keying encodes the bits of information not by on or off, but by transmitting at two different frequency for the ones and the zeros. These rapid changes in the frequency of the light can be achieved by small changes in the current applied to the laser diode. This light can be decoded by filtering the light to only allow one frequency pass. This is shown in FIG. 3. This results in a conversion to amplitude modulation which can be detected in the normal way at the receiver. The filtering can take place either after the transmitter or before the receiver, and the filter can be realised in several different ways, such as Mach-Zender interferometer or a Fabry-Perot filter. As only small changes in frequency are required (dependent upon the bit rate), the wavelength spread is minimal and the effect of dispersion is lessened.
There are many non-linearities present in transmitting optical signals along optical fibres, but of particular concern in the present invention is the Kerr effect. The Kerr effect is that the speed of transmission of light through a fibre is a function of the intensity of the light. Although this is usually only a very small effect, at sufficiently high intensities and over long enough distances the net effect can be quite dramatic, including extreme pulse narrowing and chaotic behaviour.
One known technique which utilises non-linearity to overcome the effects of dispersion is soliton systems. Solitons, or solitary waves are pulses of a mathematically defined shape (solutions of the non-linear Schrodinger Equation) which can travel along a dispersive non-linear medium without change of shape. At a particular intensity, the pulse narrowing effect of the non-linearity exactly cancels the pulse broadening effect of the dispersion and the pulse propagates undisturbed indefinitely. Generally, each soliton represents a 1 in the data stream and absence of a soliton indicates a zero.
Soliton propagation, however, requires both constant intensity of the light, and that the pulses be solitary. To achieve the latter the pulses must be separated from each other by a distance much larger than the width of each individual pulse. This requirement results from the non-linear interaction which occurs between pulses that are too close leading to chaotic behaviour. Indeed, by definition, solitons must be propagated in a solitary way.
Various researchers have investigated soliton transmission systems of the type described, for instance Mollenauer et al, IEEE Journal of Quantum Electronics, Vol QE-22, No. 1, January 1986 p 157-173. Kubota and Nakazawa, IEEE Journal of Quantum Electronics, Vol 26, No. 4, April 1990 discuss a variety of soliton propagation called pre-emphasis wherein the input power of the soliton is increased.
The various papers in the literature provide, in summary, the following rules for soliton propagation:
i/ the intensity of the pulse should not deviate by more than 4 dB from the mean intensity; and
ii/ the pulses should be separated by a distance such that the pulses are non-interacting. This in general requires a separation of at least 7 to 10 times the width of the pulse.
In soliton propagation, when the intensity of the pulse is reduced, the pulse broadens to maintain the relationship between intensity and width which is characteristic of a soliton. On the other hand, an increase in intensity narrows the pulse until the characteristic soliton relationship between width and intensity is regained. This narrowing and broadening of the pulses adds to the interaction with neighbouring pulses and so this imposes the constraint on the intensity deviation and the pulse separation. In practical terms these limitations mean that the amplifier spacing is limited to distances over which a soliton will decay by about 8 dB in practical systems. Solitons are introduced at a higher intensity and decay to a lower intensity than the required average intensity.